1. Field of the Invention
The invention concerns a commutation circuit for a p-pulse converter with controlled valves and, in particular, a commutation circuit of the type which includes an intermediate DC link which has a smoothing choke and which is connected to a controllable converter fed from a DC voltage source.
2. Description of the Prior Art
Intermediate-link converters having a smoothing choke and, thus, impressed current, in the intermediate DC link are known, for example, from the German Offenlegungsschrift 1,513,518.Such converters are used in variable-speed drives for rotating-field machines. In operation of a rotating-field machine as a motor, a converter operating as a rectifier supplies energy to the intermediate DC link from a three-phase network. The rectifier, equipped with controlled valves, receives control pulses from a control unit which is synchronized with the line voltage. A first converter, operating as an inverter, which is likewise equipped with controlled valves, is fed by the DC link and develops a rotary voltage of adjustable frequency for controlling the rotating-field machine. The inverter can be free-running or externally controlled by a control unit. The rotary voltage of the inverter develops freely, i.e., nearly independently of the waveshape of the voltage in the intermediate DC link, due to the decoupling effect of the smoothing choke arranged in the DC link. The current in the intermediate DC link remains constant during the commutation; therefore, only the DC current impressed by the smoothing choke needs to be commutated. For reversing the flow of energy, i.e., for operation as a generator, the intermediate-link voltage is reversed, with the direction of the current remaining the same. This is realized by changing the control angle of the first (on the machine side) converter, which is possible due to the absence of bypass valves.
It is also known to provide such an intermediate-link converter with a control circuit which ensures that the current in the intermediate DC link remains constant. This is taught, for example, in the German Offenlegungsschrift 1,814,400 and in "Siemens-Zeitschrift" 45, April, 1971, no. 4, pages 195 to 197 (Reprint).
In the area of intermediate-link converters which, contrary to the converters mentioned so far, do not operate with impressed DC current but with impressed DC voltage in the intermediate circuit, it is also known to form the converter from a first converter preceded by a second converter which is designed as a DC controller (See, e.g., German Offenlegungsschrift 2,050,787). The first converter, which supplies a rotating-field machine, includes controlled valves which are connected in a three-phase bridge circuit and across each of which is connected an uncontrolled bypass valve in antiparallel relationship. The DC controller comprises a controlled main valve and a quenching device which is shunted across the main valve and which includes a quenching capacitor, a controlled quenching valve and a commutating choke all connected in series. The quenching capacitor is also connected in series with a controlled recharging valve, which is fired together with the main valve, between the two inputs of the first converter. The DC controller is used in this case as the commutation circuit for the controlled valves of the first converter. If the main valve of the controller is quenched by the firing of the quenching valve, the input voltage of the first converter becomes zero, the current in the then current-carrying valves is extinguished and a bypass current begins to flow via the uncontrolled bypass valves. In an intermediate-link converter with impressed DC current in the intermediate DC link one cannot use a DC controller in the above manner (i.e., as the commutation circuit), as due to the absence of bypass valves in the first converter, the impressed intermediate-link current in the controlled main valve cannot be interrupted directly by firing the quenching valve of the DC controller.
In the area of intermediate-link converters which operate with impressed DC voltage in the intermediate link, it is also known to arrange in the intermediate link a separate commutation device which is utilized for sum-quenching of the controlled valves of the first converter (see, e.g., German Offenlegungsschrift 1,613,774). If one were to provide such a commutation device for an intermediate-link converter which operates with impressed DC current in the intermediate DC link, a considerable number of components and control devices would be required.
DC control elements (controllers) having various configurations figurations are also known from the German Auslegeschrift 1,180,833 and from the German Patent 1,242,289.
It is an object of the present invention to provide an improved intermediate-link converter of the type in which a p-pulse first converter is connected to a second controlled converter via an intermediate-link DC current. More particularly, it is an object of the present invention to provide for such an intermediate-link converter, a second controlled converter having little complexity, and, in addition, a commutation circuit for the first converter which works reliably and which can be inexpensively designed with a small number of components. It is a further object of the invention to provide a commutation circuit for the aforesaid first converter which responds quickly and is suitable for use with inductive loads, particularly loads comprising variable-speed drives of rotating-field machines.